Stabilized oscillator



L. BLOK ET AL Dec. 4, 1956 STABILIZED OSCILLATOR 2 Sheets-Sheet 1 Filed April 10, 1952 DC URCE Iso :il: INVENTORS Lourens Blok Gerri? Storre Gooitzen'Zwonenburg Jacob Willem AGE/VT Dec. 4, 1956 BLOK 2,773,192

STABILIZED OSCILLATOR Filed April 10, 1952 2 Sheets-Sheet 2 Km f4 MAGNETIC con. FORCE IN Kg.

SPRING FORCE IN Kg.

9 j J @5 3 g aid ,3 4

//VVE/V7'0/?$ Lourens Blok Gerri? Storre Gooi'rzen Zwonenburg Jacob Willem De Ruher AGE/VT United States Patent STABILIZED OSCILLATOR Lourens Blok, Gerrit Starre, Gooitzen Zwanenburg, and

Jacob Willem de Ruiter, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application April 10, 1952, Serial No. 281,642

Claims priority, application Netherlands April 18, 1951 8 Claims. (Cl. 250-36) This invention relates to high frequency furnaces wherein the high frequency energy is derived from a grid-controlled electron-tube oscillator.

In such circuit-arrangements the feed-back voltage is usually taken from the output circuit of the oscillator. With an increase in load of the high-frequency furnace the feed-back voltage supplied to the control grid decreases, which gives rise to a decrease in the efficiency of said oscillator.

In order to stabilize the excitation of the control grid, it is known to regulate the feed-back value automatically by means of a control or regulating member whose deflection is controlled by the field of a magnet coil energized by the grid direct current.

By said automatic control the grid circuit of the oscillator tube is prevented from being overloaded upon overloads; the regulation being also effective upon underloads, and as the case may be, at no-load of the high frequency furnace. Since, however, the control is based on a variation of the grid direct current, the grid circuit, in the case of normal loads, is loaded to a value lower than the permissible value.

The chief object of the present invention is to provide an improvement of such a high frequency furnace with automatic excitation control such that the oscillator tube obtains the same maximum permissible excitation voltage under all practical operating conditions.

In attaining this object, according to the invention, the curve representing the attractive force exerted magnetically on the control member by the grid direct current upon approximately maximum permissible excitation of the oscillator in accordance with the deflection of the control member, and the curve representing the spring force acting on the control member in accordance with the deflection of the control member are mirror images of one another with respect to the axis on which the deflection is plotted, so that the control member will occupy a position of equilibrium independently of its deflection.

For any point of the control range of the high frequency furnace between no-load and full load the excitation voltage set up in the grid circuit will, when the invention is utilized have a value such that the oscillator operates under optimum working conditions.

In a normal form of the magnetic circuit with the control member, the curve representing the att-ractive'force magnetically exerted on the control member will have" 'a quadratic form. Accordingly, the spring force curve must also be quadratic.

' Although any known spring construction having a quadratic spring force curve may be employed in a high frequency furnace according to the invention, it is advantageous to use a spring construction consisting of a unilaterally clamped plate spring whose free endengages the control member, adjusting screws being-provided in a longitudinal direction of the spring between the clamping point andthe point of engagement of the control member and the spring successively engaging the said adjusting screws upon deflection of the control member.

In order that the invention may be readily carried into effect, it will now be described in greater detail with reference to the accompanying drawings, given by way of example, in which corresponding elements are furnished with the same reference numerals and in which:

Fig. 1 is a schematic diagram of an embodiment of the high frequency furnace arrangement of the present invention;

Fig. 2 is a schematic diagram of the stabilizing device 5 of the high frequency furnace arrangement of Fig. 1, in side view detail;

Fig. 2a is a schematic diagram of the stabilizing device 5 of the high frequency furnace arrangement of Fig. l, in plan view detail;

Fig. 3 is a schematic diagram of a portion of the stabilizing device 5 of the high frequency furnace arrangement of Fig. 1, in detail;

Fig. 4 is a graphic presentation of magnetic coil force versus regulating member deflection, and spring force versus regulating member deflection curves; and

Fig. 5 is a schematic diagram of a modification of the high frequency furnace arrangement of Fig. 1.

In the circuit arrangement of Fig. 1, the high frequency energy is taken from a grid-controlled electron tube oscillator comprising a triode 1. The anode circuit of the tube oscillator, constituting a Colpitts circuit, comprises an oscillatory circuit 2 determining the oscillatory frequency and including a circuit coil 3 shunted by two series-connected capacitors 4 and 4, the junction of which is connected to the cathode of tube 1. One end of the anode circuit is capacitatively coupled to the anode of tube 1 and the feed-back voltage is taken from a tapping point of the circuit capacitor 4 and applied to the control grid of tube 1 by Way of a device 5, whose function is hereinafter described. The control grid of tube 1 is further connected to the grounded cathode through a leakage resistor 6. The anode of tube 1 is fed by way of a choke 7 from a source of direct current 9, shunted by a filter capacitor 8.

The current circulating in the oscillatory circuit 2 upon oscillation of the circuit arrangement is utilized for heating a work-piece 11 inserted in a load circuit, for example, a heating coil 10. The circuit 2 is inductively coupled to a. coupling coil 12 to the output terminals 13 of which the heating coil 10 is connected.

In this circuit arrangement, the excitation of the control grid is stabilized by means of a stabilizing device 5 inserted in the feed-back circuit.v This device 5 consists of a magnet coil 14 connected, in series with the leakage resistance 6, between the control grid and cathode of the tube 1, and a movable soft iron regulating member 15, which upon energization of the magnet coil by the grid direct current, is movable against the action of a spring 16 and then alters the feed-back value by adjustment of capacitors 17 and 18 coupled to the regulating member 15.

When in accordance with the present invention, the device 5 is so constructed that the curve representing the grid direct current resulting upon approximately maximum excitation of the oscillator due to the attractive force magnetically'exerted' on the regulating member 15 inaccordance with the deflection of said member, and the curve representing the spring force acting on the regulating member 15 in accordance with the deflection of said regulating member are mirror images of one another with respect to the axis on which the deflection of'said' member is plotted, said regulating member Will occupy a position of equilibrium independently of its deflection and consequently for any load withinthe control range, said'position being, however, stable, only if the grid direct current (consequently the grid'excitation) has the prescribed value.

Referring now to Figs. 2 and 2a, the stabilizing device 5 comprises an electromagnet provided with a closed yoke 20 which is secured to a mounting plate 19 and contains the magnet coil 14.

The vertical limbs of yoke 21) are provided with two openings 21 and 21' extending in the axial direction of magnet coil 14. A threaded soft iron cylindrical core 22 is screwed into the aperture 21 and surrounded by magnet coil 14. The core 22 is furnished with an axial bore. The regulating member 15 is displaceable in the other aperture 21 of the yoke 28, at its end facing the core 22, with a pin 23 which may, for example, be made from brass and is displaceable in the bore of core 22. The end of pin 23 co-operates with, and is supported in, the free end of a plate spring 16 unilaterally clamped by a corner stay 24 secured to the mounting plate 19. In order to insure a friction-free centering of the regulating member 15, the other end of said member is supported in a plate spring 26 unilaterally clamped by a corner stay 25 secured to the mounting plate 19. A number of adjusting screws 27, successively engaged by the spring 16 upon deflection to the right of the regulating member 15, are provided in the corner stay 24 between the clamping point and the point of engagement of said regulating member, in positions at right angles to said spring. The ends of the adjusting screws 27 lie on a curved line, and by simple adjustment of said screws the desired quadratic form of the curve representing the spring force acting on the regulating member 15 (which force, though to a slight degree, is also determined by the spring 26) is obtained in accordance with the deflection of said regulating member.

The deflection of the member 15 is limited in either direction by a stop 28 co-operating with the left-hand end of said member and a shoulder 30, respectively, on a pin 29 secured to said end of said regulating member.

The end of the pin 29 is articulated to a lever 31 which is also hinged to an intermediate piece 32 movable in a support 33 fitted on the mounting plate 19. The intermediate piece 32 preferably consists of electrically conductive material and is preferably insulated from the support '33 and the lever 31. Rods 35 and 36 of conductive material connected to movable plates of the capacitors 17, 18 (Fig. 1) are articulated to the inter mediate piece 32 on either side of its center of rotation 34.

A suitable form of the capacitors 17 and 18 in view of a small deflection stroke, which capacitors are differential capacitors, is diagrammatically shown in Fig. 3. The differential capacitor consists of two sets of stationary and two sets of movable parallel capacitor plates; plates mutually movable in opposite directions being arranged between every two successive stationary plates pertaining to one set.

The two sets of stationary plates are designated 37, 37', 37", and 38, 38', 38" respectively; the two sets of movable plates being denoted 40, 40, 40" and 41, 41', 41", respectively. In Fig. 1 the plates 37, 37, 37" (capacitor 18) are connected to the tapping of the circuit capacitor 4' from which the feed-back voltage is taken. This is indicated by the alternating voltage generator 39 in Fig. 3.

The stationary plates 38, 38, 38" represent the stationary plate of the capacitor designated 17 in Fig. 1, which plates are connected to the grounded cathode of oscillator tube 1.

The two sets of plates 40, 40', 40", and 41, 41', 41" are mechanically and electrically connected respectively to the rotary intermediate piece 32 through the rods 35 and 36, respectively, said intermediate piece being electrically connected to the control grid of tube 1.

Fundamentally, the voltage divider 17, 18 may, of course, be a differential reactor of normal construction; for example, a differential rotary capacitor or a differential inductor. In practice it has been found ad.-

4 vantageous to use a capacitative voltage divider in the feed-back circuit, since the use of inductors may involve undesirable phase displacements in said feed-back circuit. This is notably at high operating frequencies of, for example, 1 megacycle per second, owing to which the anode alternating voltage and current may not accurately be in phase opposition, thus rendering the anode-dissipation unfavorable, notably upon underloads of the generator, and increasing the direct current power consumed by the tube. As an alternative, for example, for feed-back control, the circuit capacitor 4 (Fig. 1) may be variable (provided the detuning of the circuit then occurring is not objectionable). The capacitor 4 may be adjusted by the regulating member 15 in combination with the spring 16, in which case the voltage divider 17, 18 may be dispensed with, as is explained with reference to Fig. 5.

The operation of the circuit-arrangements referred to may be explained with reference to the curves of Fig. 4. Curve a represents the tractive force Km exerted by the magnet coil 14 on the regulating member 15 due to the (constant) grid direct current produced upon maximum permissible excitation of the oscillator tube 1 in accordance with the deflection d of said regulating member. The spacing d, i. e., the airgap between the core 22 and the end of the regulating member 15, is depicted in Fig. 2. The sensitivity of the electromagnet is controllable by adjustment of the core 22. With this curve plotted in a practical device, the magnet force is indicated in kg. and the deflection d in mm. at a grid direct current of 0.65 A. The construction of springs 16, 26, in cornbination with the adjustment of screws 27, is such that the curve representing the spring force Kv acting on the regulating member 15 in accordance with the deflection d of said regulating member, has a quadratic form illustrated by curve v in Fig. 4. The spring force Kv is plotted in kg. on the ordinate below the axis on which the deflection of the regulating member is plotted.

If, as illustrated by Fig. 2, by adjustment of. screws 27 the curve v representing the spring force is the mirror image of curve a relatively -to the d-axis, the regulating member 15 will, upon any load, be deflected such that the differential capacitor coupled to said regulating member is adjusted as required for the production of a grid direct current of 0.65 A.

Fig. 5 is a simplified circuit-arrangement for a highfrequency furnace of the type of Fig. 1, which may be employed particularly for comparatively low powers, for example, up to 10 kw. In Figs. 1 and 5, corresponding elements are provided with the same reference numerals.

The simplification consists primarily in the differential capacitor 17, 18 being dispensed with, and the value of the feed-back voltage taken from the end of the frequency-determining circuit 2 remote from the anode and supplied to the control grid of oscillator tube 1 is controlled by adjustment of either of the circuit capacitors, namely, capacitor 4. The adjustment of capacitor 4 is controlled by means of the regulating member 15 cooperating with the spring 16 in the manner of Fig. 1 and in accordance with the curves of Fig. 4.

It is to be understood that the invention is not limited to the details disclosed but includes all such variations and modifications as fall within the spirit of the invention and the scope of the appended claims.

What we claim is:

1. A high frequency wave generator for a furnace comprising an oscillator provided with an electron discharge tube having a control grid and an anode and circuits therefor, an output circuit coupled to said anode and a feedback circuit coupling said output circuit to the grid circuit of said tube to sustain oscillations, and a stabilizing device for said oscillator including an electromagnet having a coil interposed in said grid circuit and a control member positioned for deflection by said coil in accordance with direct current flow in said grid circuit,

arran e said control'member being coupled l said'fe'edback circuit'to vary the extent of said feedback as a function of grid current flow, and a spring element loading said conttol'member', said electromagnet having a predetermined characteristic curve representing the attractive force exerted magnetically on said control member by the direct current flow in said grid circuit in'thecondition of approximately maximum permissibleoscillator*excita= tion in accordance with defiecti'on'of 'saidcontrol member, said spring element having a givencurverepresenting'the spring force acting on said control member in accordance with deflection thereof which is a mirror image ofsaid predetermined curve with respect to" th'e'axis on which said deflection is plotted, wherebysaid control member occupies a position of equilibrium'in'dependently"of"its deflection.

2, A high frequency wave generator for a furnace comprising an oscillator provided with an electron discharge tube having a control grid and an 'anode and circuits therefor, an output circuit coupled to said anode and a feedback circuit coupling said output circuit to the grid "circuit of said tube to sustain oscillationsyand 'a stabilizing device for said oscillator including an electromagnet having a coil interposed in said grid circuit and a control member positioned for deflection by said coil in accordance with direct current flow in said grid circuit, said control member being coupled to said feedback circuit to vary the extent of said feedback, and a spring element loading said control member, said electromagnet having a predetermined characteristic curve representing the attractive force exerted magnetically on said control member by the direct current flow in said grid circuit in the condition of approximately maximum permissible oscillator excitation in accordance with deflection'of said control member, said spring having a given curve representing the spring force acting on said control member in accord ance with-deflection thereof which is a mirror image of said predetermined curve with respect to the axison which said deflection is plotted, whereby said control member occupies a position of equilibrium independently of its deflection, said spring element having a characteristic at which said curve representing the spring force acting on said control member in accordance with said deflection possesses a quadratic form.

3. A high frequency wave generator fora furnace comprising an oscillator provided with an electron discharge tube having a control grid and an anode and circuits therefor, an output circuitcoupled to said anode and a feedback circuit coupling'said output circuit to the-grid circuit of said tube to sustain oscillations, and a stabilizing device for said oscillator including an electromagnet having 'a coil interposed in said grid circuit and a control member positioned for deflection by said coil in accordance with direct current flow in said grid circuit, said control member being coupled to said feedback circuit to vary the extent of said feedback, and a spring element loading said control member at one end thereof, said electromagnet coil being provided with a closed magnetic yoke having opposing limbs with apertures therein extending in the axial direction of said coil, said control member being constituted by a first iron core having a non-magnetic pin secured thereto, said first core being displaceably positioned in the aperture of the yoke facing away from said spring element, a second iron core positioned in the other aperture of the yoke and having an axial bore, the pin of said first core extending through the bore of said second core to engage said spring element, said electromagnet having a predetermined characteristic curve representing the attractive force exerted magnetically on said control member by the direct current flow in said grid circuit in the condition of approximately maximum permissible oscillator excitation in accordance with deflection of said control member, said spring having a given curve representing the spring force acting on said control member in accordance with deflection thereof which is a mirror image of said predetermined curve with 6 respect 'to tli'eaxis (in-amassindflec'tion" 's plo'ttedi whereby said control ineniber'oc'c pies a position of equb librium independentlypf its deflection.- v 4. A high frequency-wave generatorfor'a fiirriace comprising -aa---os i11ardr provided with an--electr'on discuits "therefor;- an" output "circuit coupled to said-anodecomprising 'a resonant network including "a 5 variable capacitor and a" feedback circuiteoapungsaid "output cir'-* cuit to the grid circuit of-saidttib to sustain oscillations; and a stabilizing'devicefor-said oscillator'"incliiding-an electromagnet 'liaving'a Coil interposed in said 1 grid citcuit and ai'co'ntrol member-warranted for deflection by said "c'oili'rr accordance with direct 'currentflow' in said grid circuit, said control member being coupled T to said variable capacitor to vary the capacitance value thereof'and thereby to vary the extent of said feedback as {a function of 'grid current flow, anda spring element loading-said control member, said electromagnet having a predete'r mined characteristic curve representing ythe attractive forcet'exerted magnetically on said "control-member by the direct 'current'fiow in said grid' circuit in the-condition of approximately maximum permissibleoscillator excitation in accordance with deflection of said control menibensaid spring element having a given curve repre senting the spring-force acting-on saideontrol member in accord'ance witlr the deflection thereof "whichis a mirror'image of said predetermined curve-with respect-to the axis on which said deflection-is plotted, whereby said control member occupies a position 'of equilibrium inde pendently of its deflection;

5. A high frequency wavegene'ratof fora furnace comprising an oscillator provided'-witlr--an electrondischarge tube having ac'ontrolgrid and an anodeand 'cir cuits th'erefor; an output circuit coupl'ed 't'o' said anode and a feedbackcircuit coupling said output circuit "to the grid circuit of said tube-t0 sustain oscillations, saidife'ed back circuitcompris'in'ga voltagedivider formed by a differential capacitor, anda stabilizingdevice -fo'r"said oscillator including an electromagnet having'a coil-inter posed in said grid-circuit and a control -memb'erposi tioned for deflection by said 'coil inaccordance -'with' direct current flow in said grid circuit, said control member being coupled to said -differential capacitor tovary the extent 'of saidfeedba'ck as afunction of grid current new, and a spring element' loading saidcontrol-member,'said electromagnet having apredetermined characteristic curve representing the attractive-force exertewmagnetically onsaidcontrol member'by the direct current flow in said grid circuit in the condition of approximately maximum permissible oscillator excitation in accordance with deflection of said control member, said spring element having a given curve representing the spring force acting on said control member in accordance with deflection thereof which is a mirror image of said predetermined curve with respect to the axis on which said deflection is plotted, whereby said control member occupies a position of equilibrium independently of its deflectron.

6. A high frequency wave generator for a furnace comprising an oscillator provided with an electron discharge tube having a control grid and an anode and circuits therefor, an output circuit coupled to said anode and a feedback circuit coupling said output circuit to the grid circuit of said tube to sustain oscillations, said feedback circuit comprising a voltage divider formed by a differential capacitor comprising two sets of stationary plates, two sets of movable parallel plates, two movable plates being positioned between every two successive stationary plates of one of said two sets of stationary plates, a stationary plate of the second of said two sets of stationary platesbeing positioned between said two movable plates, and means for moving said two movable plates in mutually opposing directions and at right angles to said stationaryplates, and a stabilizing device for said oscillator includ-- ing an electromagnet having a coil interposed in said grid circuit and a control member positioned for deflection by said coil in accordance with direct current flow in said grid circuit, said control member being coupled to said last-mentioned means of said differential capacitor to vary the extent of said feedback as a function of grid current flow, and a spring element loading said control member, said electromagnet having a predetermined characteristic curve representing the attractive force exerted magnetically on said control member by the direct current flow in said grid circuit in the condition of approximately maximum permissible oscillator excitation in accordance with deflection of said control member, said spring element having a given curve representing the spring force acting on said control member in accordance with deflection thereof which is a mirror image of said predetermined curve with respect to the axis on which said deflection is plotted, whereby said control member occupies a position of equilibrium independently of its deflection.

7. A high frequency wave generator for a furnace comprising an oscillator provided with an electron discharge tube having a control grid and an anode and circuits therefor, an output circuit coupled to said anode and a feedback circuit coupling said output circuit to the grid circuit of said tube to sustain oscillations, and a stabilizing device for said oscillator including an electromagnet having a coil interposed in said grid circuit and a control member positioned for deflection by said coil in accordance with direct current flow in said grid circuit, said control member being coupled to said feedback circuit to vary the extent of said feedback, and a spring element loading said control member comprising a unilaterally clamped plate spring having a free end in engagement with said control member, and a plurality of adjusting screws positioned between the fixed end of said plate spring and said free end of said plate spring in successive engagement with saidplate spring in response to deflection of said control member, said electromagnet having a predetermined characteristic curve representing the attractive force exerted magnetically on said control member by the direct current flow in said grid circuit in the condition of approximately maximum permissible oscillator excitation in accordance with deflection of said control member, said spring element having a given curve representing the spring force acting on said control member in accordance with deflection thereof which is a mirror image of said predetermined curve with respect to the axis on which said deflection is plotted, whereby said control member occupies a position of equilibrium independently 8 of its deflection, said spring element having a characteristic at which said curve representing the spring force acting on said control member in accordance with said deflection possesses a quadratic form determined by the adjustment of said adjusting screws.

8. A high frequency wave generator for a furnace comprising an oscillator provided with an electron discharge tube having a control grid and an anode and circuits therefor, an output circuit coupled to said anode and a feedback circuit coupling said output circuit to the grid circuit of said tube to sustain oscillations, and a stabilizing device for said oscillator including an electromagnet having a coil interposed in said grid circuit and a control member positioned for deflection by said coil in accordance with direct current flow in said grid circuit, said control member being coupled to said feedback circuit to vary the extent of said feedback, a spring element loading said control member comprising a unilaterally clamped plate spring having a free end in engagement with said control member, and a plurality of adjusting screws positioned between the fixed end of said plate spring and said free end of said plate spring in successive engagement with said plate spring in response to deflection of said control member, and means for supporting said control member for substantially friction-free centering comprising a second unilaterally clamped plate spring having a free end in engagement with said control member, said electromagnet having a predetermined characteristic curve representing the attractive force exerted magnetically on said control member by the direct current flow in said grid circuit in the condition of approximately maximum permissible oscillator excitation in accordance with deflection of said control member, said spring element having a given curve representing the spring force acting on said control member in accordance with deflection thereof which is a mirror image of said predetermined curve with respect to the axis on which'said deflection is plotted, whereby said control member occupies a position of equilibrium independently of its deflection, said spring element having a characteristic at which said curve representing the spring force acting on said control member in accordance with said deflection possesses a quadratic form determined by the adjustment of said adjusting screws.

References Cited in the file of this patent UNITED STATES PATENTS 

